Point moonlock and moonset at the wallpaper store path

Both fall back to /usr/share/moonarch/wallpaper.jpg, which does not exist
under Nix, so they rendered without a background.

modules/desktop.nix

@@ -82,6 +82,16 @@ in
     # stasis reads /etc/stasis/ as its system-wide fallback, never /etc/xdg/.
     "stasis/stasis.rune".source = "${xdg}/stasis/stasis.rune";
 
+    # moonlock and moonset fall back to /usr/share/moonarch/wallpaper.jpg,
+    # which does not exist here, so the wallpaper is pointed at the store path
+    # explicitly. Both read these files as their system-wide config.
+    "moonlock/moonlock.toml".text = ''
+      background_path = "${wallpaper}"
+    '';
+    "moonset/moonset.toml".text = ''
+      background_path = "${wallpaper}"
+    '';
+
     # Configs outside /etc/xdg/ in the Arch package as well.
     "mpv/mpv.conf".source = "${etc}/mpv/mpv.conf";
     "xdg-desktop-portal/niri-portals.conf".source =